Denitration catalyst, process for preparing the same, and exhaust gas purification method

ABSTRACT

A denitration catalyst for use in the reduction of nitrogen oxides contained in an exhaust gas containing highly deliquescent salts as dust with ammonia, which bears thereon a porous coating of a water-repellent organic resin, a porous coating of a mixture of a water-repellent organic resin with inorganic oxide particles, or a porous coating of a mixture of a water-repellent organic resin with catalyst component particles. The denitration catalyst can be prepared, for example, by coating the surface of a denitration catalyst with an aqueous dispersion containing a water-repellent organic resin having a lower concentration, drying the coating, further coating the dried coating with an aqueous dispersion containing a water-repellent organic resin having a higher concentration, and then drying the coating to form a porous coating of a water-repellent organic resin. This denitration catalyst, even when used in the treatment of an exhaust gas containing ash containing highly deliquescent salts, enables the water-repellent porous coating formed on the surface to prevent the salts that have deliquesced from entering the catalyst. This can prevent the deterioration of the catalyst and enables a high catalytic activity to be maintained for a long period of time.

TECHNICAL FIELD

The present invention relates to a denitration catalyst, a process forpreparing the same, and an exhaust gas purification method, and inparticular to a denitration catalyst for efficient catalytic reduction,with ammonia (NH₃), of nitrogen oxides (NOx) contained in an exhaust gascontaining highly deliquescent salts as dust, such as in an oil-firedboiler exhaust gas, a combustion exhaust gas from wood waste, wood asfuels, and in a furnace for refuse.

BACKGROUND ART

NOx in flue gas discharged from a power station or an incinerator is asubstance causing photochemical smog and acid rain, and a flue gasdenitration method by selective catalytic reduction with ammonia as areducing agent is used widely as a method for effective removal of NOxmainly in thermal power stations. As the catalyst, a titanium oxide(TiO₂) type catalyst containing vanadium (V), molybdenum (Mo) ortungsten (W) as the active component is used, and particularly thecatalyst containing vanadium as one of the active components is highlyreactive and can be used at low temperatures (e.g. in a temperaturerange of 300° C. or less), so it becomes the mainstream of thedenitration catalyst at present (Japanese Patent Application Laid-OpenNo. 50-128681A and the like).

The catalyst in the prior art mentioned above has superiorcharacteristics by which very high degrees of denitration can beachieved in purification of combustion exhaust gases such as gas-fired,oil-fired or coal-fired fuel exhaust gases, but no adequate measureshave been taken to purify an exhaust gas from wood waste and wood asfuels abundant in North Europe or the like area and an exhaust gascontaining a large amount of deliquescent salts in ash from anincinerator and the like, and there has been the problem that thedegrees of denitration are lowered with time.

FIG. 7 shows the degrees of denitration and the change of the amount ofan alkali accumulated in the conventional denitration catalyst in thecase (A) where the conventional catalyst was exposed to a wood-firedboiler combustion exhaust gas as an example of application to an exhaustgas containing a large amount of deliquescent salts and the case (B)where the catalyst was exposed to a coal-fired boiler exhaust gas as anexample not containing deliquescent salts. In the case (A) of thewood-fired boiler containing a large amount of potassium carbonate as adeliquescent alkali metal salt, there occurs the phenomenon in which theamount of the alkali in the catalyst is increased and the degree ofdenitration rapidly decreases.

Such deterioration of the denitration catalyst by the alkali metal saltalso occurs where an incinerator exhaust gas or a high-sulfur oil-firedboiler exhaust gas is to be denitrated, thus greatly preventingpractical application of the low-temperature denitration method to suchexhaust gases.

It was found that the above deterioration by the highly deliquescentsalt is caused by inclusion, in ash, of (1) potassium carbonate in thecase where wood waste is used as the fuel, (2) calcium chloride orsodium chloride in the case of refuse combustion exhaust gas, and (3)sodium sulfate and potassium sulfate in the case of high-sulfuroil-fired boilers. Accordingly, it is believed that although the alkalimetal salts in ash differ depending on the type of exhaust gas asdescribed above, any deterioration of the catalyst duringlow-temperature denitration is caused by the common mechanism in whichthe alkali metals and alkaline earth metal salts contained in ash absorbmoisture to deliquesce to form a liquid in the step of temperatureraising when a denitration apparatus starts or stops, so that the fluidof the metal salts penetrates into the catalyst to cause the clogging ofpores therein and to deteriorate the active site.

To prevent the deterioration of the denitration catalyst by such fluidhaving deliquesced, it is necessary to physically prevent the fluid thathas deliquesced from entering the denitration catalyst.

The present invention provides a highly durable denitration catalystwhich is prevented from undergoing rapid deterioration upondeliquescence of salts, which deterioration occurs in the case ofdenitration of an exhaust gas containing ash having a wide variety ofdeliquescent salts as described above, a process for preparing the thedenitration catalyst, and a method for purification of an exhaust gas byuse of the same.

DISCLOSURE OF THE INVENTION

The inventions claimed in this patent application are as follows:

(1) A denitration catalyst for use in the reduction, with ammonia, ofnitrogen oxides contained in an exhaust gas containing highlydeliquescent salts as dust, which bears on the surface of thedenitration catalyst a porous coating layer of a water-repellent organicresin, a porous coating layer of a mixtures of a water-repellent organicresin with inorganic oxide particles, or a porous coating layer of amixture of a water-repellent organic resin with catalyst componentparticles.

(2) A denitration catalyst according to (1), wherein the water-repellentorganic resin comprises at least one of fluorine resin, polyamide resin,acrylic resin and silicon resin.

(3) A denitration catalyst according to (1) or (2), wherein the catalystcomponent particles consist of titanium oxide, molybdenum oxide ortungsten oxide and vanadium oxide.

(4) A denitration catalyst according to any one of (1) to (3), whereinthe denitration catalyst is a catalyst molded body containing titaniumoxide, molybdenum oxide or tungsten oxide, and vanadium oxide.

(5) A denitration catalyst according to any one of (1) to (3), whereinthe denitration catalyst is a plate-shaped molded body comprising acomposition mainly composed of titanium oxide and molybdenum oxide ortungsten oxide, and vanadium oxide, filled between network inorganicfiber substrates and within the nets thereof.

(6) A process for preparing a denitration catalyst, which comprisespermitting an aqueous slurry or emulsion of at least one resincomposition selected from the group consisting of a water-repellentorganic resin, a mixture of the water-repellent organic resin andinorganic oxide particles, and a mixture of the water-repellent organicresin and catalyst component particles to be coated on, or to adhere bya spray to the surface of a denitration catalyst and then evaporatingits water to form their porous coating layer.

(7) A process for preparing a denitration catalyst according to (6),wherein the denitration catalyst is a catalyst molded body containingtitanium oxide and molybdenum oxide or tungsten oxide, and vanadiumoxide, and the catalyst molded body is previously moistened to form theporous coating layer in a wet state.

(8) A process for preparing a denitration catalyst, which comprisescoating an aqueous dispersion containing a water-repellent organic resinat a low concentration onto the surface of a denitration catalyst foruse in the ammonia reduction of nitrogen oxides contained in an exhaustgas containing highly deliquescent salts as dust, drying the coating,further coating the dried coating with an aqueous dispersion containinga water-repellent organic resin at a higher concentration than the aboveconcentration, and then drying the coating to form a porous coatinglayer of the water-repellent organic resin.

(9) A process for preparing a denitration catalyst according to (8),wherein the concentration of the water-repellent organic resin in thelow-concentration aqueous dispersion ranges from 5 to 30% by weight.

(10) A process for preparing a denitration catalyst according to (8),wherein the concentration of the water-repellent organic resin in thelow-concentration aqueous dispersion ranges from 10 to 20% by weight.

(11) A process for preparing a denitration catalyst according to (8),wherein the concentration of the water-repellent organic resin in thehigh-concentration aqueous dispersion ranges from 30 to 60% by weight.

(12) A process for preparing a denitration catalyst according to any oneof (8) to (11), wherein the water-repellent organic resin comprises atleast one of fluorine resin, polyamide resin, acrylic resin and siliconresin.

(13) A process for preparing a denitration catalyst according to any oneof (8) to (12), wherein the denitration catalyst is a catalyst moldedbody containing titanium oxide molybdenum oxide or tungsten oxide andvanadium oxide.

(14) A denitration catalyst according to any one of (8) to (12), whereinthe denitration catalyst is a plate-shaped molded body comprising acomposition mainly composed of titanium oxide and molybdenum oxide ortungsten oxide, and vanadium oxide, having filled between networkinorganic fiber substrates and within the nets thereof.

(15) A denitration catalyst prepared by a process according to any oneof (8) to (14).

(16) A method for purification of an exhaust gas, which comprisesreduction, with ammonia, of nitrogen oxides contained in an exhaust gascontaining highly deliquescent salts as dust by use of a denitrationcatalyst according to any one of (1) to (15).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the working effect of the denitrationcatalyst of the present invention;

FIG. 2 is a drawing showing the working effect of a conventionalcatalyst;

FIG. 3 is a drawing showing the surface of the denitration catalystaccording to one example of the present invention;

FIG. 4 is a drawing showing the surface of the denitration catalystcoated with a low-concentration dispersion;

FIG. 5 is a drawing showing the denitration catalyst coated with ahigh-concentration dispersion;

FIG. 6 is a drawing showing the surface of the denitration catalystcoated repeatedly with the low-concentration dispersion; and

FIG. 7 is a drawing showing the problem of the conventional catalyst.

In the drawings, 1 means the denitration catalyst component layer; 2, awater-repellent coating layer; 3, an alkali that has deliquesced; 4,denitration catalyst particles; 10, the denitration catalyst; 11, thesurface of the catalyst; 12 and 12a, cracks; 13, 13a, 14 and 14a,coating layers; and 16, 16a, 17 and 18, penetration layers.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention is described in detail by referenceto the drawings.

FIG. 1 is a drawing showing the working effect of the denitrationcatalyst of the present invention, and FIG. 2 is a drawing showing theworking effect of a conventional catalyst.

In FIG. 1, the denitration catalyst of the present invention bears, onthe surface of the denitration catalyst component layer 1 consisting ofthe denitration catalyst particles 4, the water-repellent coating 2selected from a porous coating layer of a water-repellent organic resin,a porous coating layer of a mixture of a water-repellent organic resinwith inorganic oxide particles, or a porous coating layer of a mixtureof a water-repellent organic resin with catalyst component particles.

In general, when the denitration catalyst is used in an exhaust gascontaining highly deliquescent alkali metal salts, dust adheres asparticles to the surface of the catalyst.

The alkali metal salts adhered to the catalyst absorb moisture in theatmosphere or exhaust gas to deliquesce and enter the catalyst when theapparatus stops or starts. When the alkali salts adhered to the surfaceof the denitration catalyst component layer 1 deliquesce, alkali 3deliquesced penetrates through the spaces among the denitration catalystparticles 4 into the inside of the catalyst as shown in FIG. 2, thuscausing the clogging of pores and simultaneously reacting with thecatalyst component to denature the active site and to deteriorate thecatalyst. By way of example, the conventional catalyst is embedded inpotassium carbonate, and after maintained for one day under highhumidity, the catalyst was observed for the outer appearance, and as aresult it was found that the potassium carbonate deliquesced and adheredto the surface of the catalyst after the test. The catalyst was dried,the salt deliquesced was removed therefrom, and the amount of potassiumin the catalyst was determined. The result indicated that the potassiumaccounted for several tens % by weight, and it was thus found that thepotassium carbonate entered the inside of the catalyst.

When the conventional catalyst is used in an exhaust gas containingdeliquescent salts as dust, the dust having adhered thereto deliquesceswith water when the denitration apparatus stops or starts again, and thefluid of the dust transfers into the catalyst to cause the reduction ofthe activity of the catalyst.

To the contrary, the denitration catalyst according to the presentinvention bears the water-repellent coating layer 2 on the surface ofthe denitration catalyst component layer 1, and this water-repellentcoating layer 2 has a large contact angle to water, salts, so even ifthe alkali 3 deliquesced becomes fluid to adhere to the surface of thecatalyst, it is repelled as shown in FIG. 1. Further, because of itssurface tension, the deliquescent substance cannot penetrate into theinside of the catalyst so it does not cause clogging of pores ordeterioration of the catalyst resulting from denaturation of the activesite. Further, the water-repellent coating 2 is porous, repels fluidbecause of its surface tension, but can pass gases such as reactive gas,vapor through it, so it does not prevent the denitration reaction.

On the other hand, the high water-repellent effect of the surface of thecatalyst is achieved by forming a water-repellent coating layer having asufficient thickness on the surface of the catalyst, but if the coatinglayer is too thick, the water-repellent effect is raised, but thediffusion of an exhaust gas into the catalyst is prevented, so thedenitration performance of the catalyst is easily lowered, and further alarge amount of the water-repellent component coated leads to highercosts. Accordingly, it is preferable to reduce the amount of thewater-repellent component coated on the surface of the denitrationcatalyst. Further, if the surface of the catalyst has a site where acoating layer of the water-repellent component is not formed, the fluidof the alkali deliquesced penetrates through such sites to cause thedeterioration of the catalyst, so it is preferable to form a thin anddense coating.

If a dispersion of a high-concentration water-repellent component isused to form the water-repellent coating layer on the surface of thecatalyst, a thin and uniform water-repellent coating cannot be appliedon the surface of the catalyst. Further, if a low-concentrationdispersion is used, there occurs a site where a water-repellent coatinglayer is not formed on the surface of the catalyst and this site servesas a path through which the alkali penetrates, so there is the casewhere the durability of the catalyst for a long period of time cannot beachieved.

FIG. 4 is a drawing showing the surface of the catalyst coated with alow-concentration dispersion.

In FIG. 4, when the low-concentration dispersion is coated on thesurface of the catalyst of the denitration catalyst 10, fine particlesof the water-repellent component, along with the dispersion medium,penetrate through the catalyst surface 11 and cracks 12 on the surfaceto the inside of the catalyst, to form the penetration layer 16 in thecatalyst. Further, the coating layer 13 is formed on the surface of thecatalyst 11, but the coating layer 13 is thin and the amount of thewater-repellent component carried is small, so the water-repellentcoating layer is not formed in crack 12 on the surface of the catalyst.Accordingly, the sufficient water-repellent effect of thewater-repellent coating layer cannot be obtained, and the durability ofthe denitration catalyst 10 is lowered.

FIG. 5 is a drawing showing the surface of the catalyst coated with ahigh-concentration dispersion.

In FIG. 5, when the catalyst surface 11 of the denitration catalyst 10is coated with a high-conc. dispersion, the penetration layer 17 isformed in the catalyst, and the dispersion medium selectively penetratesinto the catalyst to form the thick coating layer 14a on the surface ofthe catalyst 11, and further the cracks 12 on the surface of thecatalyst are filled with the water-repellent component. However, becausethe coating layer 14a is thick, the diffusion of an exhaust gas into thecatalyst is prevented and the denitration activity is decreased.Further, the cracks 12a occur in the coating layer 14a itself, and thecracks 12a serve as paths through which the fluid deliquesced passes,thus decreasing the durability of the denitration catalyst 10.

FIG. 6 is a drawing showing the surface of the catalyst coatedrepeatedly with a low-concentration dispersion.

In FIG. 6, the surface of the catalyst 11 of the denitration catalyst 10is coated with a low-concentration dispersion to form a thinwater-repellent coating layer, and thereafter, it is further coated withthe low-concentration dispersion to form the coating layer 13a. However,even if the catalyst is coated repeatedly with the low-concentrationdispersion, the dispersion penetrates through the crack 12 on thesurface of the catalyst 11 into the inside of the catalyst to form thehigh-concentration penetration layer 18 in the catalyst, but the coatinglayer 13a cannot be formed in the cracks 2 on the surface of thecatalyst.

The present invention prevents the occurrence of the problem describedabove, and it is preferable for improvement of the durability of thecatalyst that an aqueous dispersion containing a water-repellent organicresin at a low concentration is coated and dried on the surface of thedenitration catalyst used for ammonia reduction of nitrogen oxidescontained in an exhaust gas containing deliquescent salts as dust, andthen an aqueous suspension containing the water-repellent organic resinat a higher concentration than the above concentration is further coatedand dried thereon to form a porous coating layer of the water-repellentorganic resin.

FIG. 3 is a drawing showing the surface of the denitration catalystproduced by the process described above.

In FIG. 3, a low-concentration dispersion is first coated and dried onthe surface of the catalyst 11 of the denitration catalyst 10 to formthe thin water-repellent coating layer 13. In this step, the cracks 12on the surface of the catalyst 11 are not coated with the coating layer13 (see FIG. 4). Then, the surface of the catalyst on which the coating13 was formed is coated with a high-concentration dispersion whereby thecoating layer 14 is formed. In this step, the dispersion medium for thehigh-concentration dispersion penetrates through the crack 12 and a thinportion of the coating layer 13 into the catalyst to form thepenetration layer 16a, and fine particles of the water-repellentcomponent in the dispersion are accumulated on the coating layer 13 andthe crack 12 on which the low-concentration dispersion could not beformed, to form the coating layer 14.

In this manner, after the thin water-repellent coating on the surface ofthe catalyst is formed from the low-concentration dispersion, then thehigh-concentration dispersion is coated thereon whereby the selectiveabsorption, into the catalyst, of only the dispersion medium occurringon coating the high-concentration dispersion can be prevented, and thethin coating layer can thereby be formed from the high-concentrationdispersion. In this case, it is important to confer water repellence onthe surface of the catalyst by coating the surface with thelow-concentration dispersion and drying it. By coating thehigh-concentration dispersion on the water-repellent coating layer, thedispersion medium is absorbed through cracks and thin portions of theinitially formed coating layer, and simultaneously the water-repellentcomponent is selectively accumulated on such portions, to form awater-repellent aqueous coating, thus reducing the non-water-repellentportion and forming the uniform and thin coating layer whereby thehighly durable catalyst can be obtained.

In the present invention, the water-repellent coating layer can be givenby drying the dispersion after coated, or by further calcination at atemperature of 100 to 380° C. after drying. The means for coating thedispersion are not particularly limited, but can be effected by anymeans known in the art. For example, mention is made of those by meansof brush, roller, spray and the like or impregnation of the catalystwith the dispersion.

In the present invention, the denitration catalyst is preparedpreferably by forming a composition containing titanium oxide, tungstenoxide or molybdenum oxide, and vanadium oxide into a molded body in theform of plate, honeycomb, particle and the like. In particular, adenitration catalyst of a plate-shaped molded body comprising acomposition mainly composed of titanium oxide and molybdenum oxide ortungsten oxide, and vanadium oxide filled between network inorganicfiber substrates and within the nets thereof is preferably used.

The porous coating layer of a water-repellent organic resin is formedpreferably by coating or spraying a slurry having fine particles of awater-repellent organic resin selected from fluorine resin, polyamideresin, silicon resin and acrylic resin dispersed in water, or by coatingor spraying a slurry having fine particles of the water-repellentorganic resin and fine particles of inorganic oxides such as silica,titania dispersed together therein, or a slurry having fine particles ofthe water-repellent organic resin and catalyst component particlesdispersed therein. The term "fine particles" refers not only tospherical fine particles but also to fine fibrous materials such asfibrils. The size of the fine particles is preferably 500 Å or more soas not to penetrate into fine pores in the catalyst to cause clogging ofthe pores.

In the present invention, the catalyst component particles in the porouscoating layer include particles consisting of a mixture of titaniumoxide and tungsten oxide or molybdenum oxide, and vanadium oxide.Further, in the present invention, the catalyst component layer on whichthe coating layer is formed is designed to have a two-layer structurewhose surface layer is a vanadium compound-free catalyst component layerto further prevent it from being liable to the toxicity of alkali metalsalts.

Formation of the coating layer is conducted usually after calcination ofthe catalyst, which may be either in a dry state or in a wet state aftermoistened. In a wet state, there are the advantages that penetration ofthe water-repellent organic resin into the catalyst pores can beprevented due to water in the catalyst pores and that the amount of thewater-repellent organic resin necessary for the coating is reduced. Evenif the coating layer is formed on the surface of the catalyst, thiscoating layer is porous, repels a deliquescence fluid due to its surfacetension, and can pass gases such as exhaust gas and vapor through it, sothis coating layer does not prevent the denitration reaction.

In the present invention, when a low-concentration dispersion ispreviously coated to form a porous coating layer, the concentration of awater-repellent organic resin in the dispersion is preferably 5 to 30%by weight, more preferably 10 to 20% by weight for coating propertiesand from an economical viewpoint. The concentration of thewater-repellent organic resin in the high-concentration dispersionsubsequently coated is not particularly limited insofar as it is higherthan the concentration of the low-concentration dispersion, and itsconcentration is preferably 30 to 60% by weight for coating properties,coating uniformity and the like. It is preferable to select theconcentration suitably depending on coating conditions andwater-repellence required of the catalyst surface of the catalyst.Further, the dispersion can contain other components in such a range asnot to impair the object of the present invention.

According to the denitration catalyst of the present invention, even ifdeliquescent salts absorb moisture and deliquesce in the step oftemperature raising when a denitration apparatus starts or stops, theporous coating layer of the water-repellent organic resin on the surfacerepels the salts in a liquid form to prevent them from entering thecatalyst whereby the catalyst is prevented from deterioration and theactivity thereof can thereby be maintained for a prolonged period oftime.

The temperature for use of the catalyst of the present inventionsubjected to water-repellent treatment is the temperature at which thewater-repellent organic resin is not molten or burned; for example, ifpolytetrafluoroethylene is used as the fluorine resin, 350° C. or lessis preferable.

Hereinafter, the present invention is described in more detail byreference to the Examples. In the Examples, "%" means "% by weight"unless otherwise specified.

EXAMPLE 1

2.5 kg of ammonium molybdate ((NH₄)₆ Mo₇ O₂₄ /4H₂ O)), 2.33 kg ofammonium methavanadate, 3.0 kg of oxalic acid, 4.8 kg of inorganicfibers (trade name: Kaowool) and water were added to 20 kg of titaniumoxide powder, and these were kneaded in a kneader to prepare a substratepaste with a water content of 33%.

On the other hand, a network material having 1400 twisted filaments of Eglass fibers with a fiber diameter of 9 μm plain-woven at a roughness of10 filaments/inch was impregnated with a slurry consisting of 40% oftitania, 20% of silica sol and 1% of polyvinyl alcohol, and dried at150° C. to make it rigid to give a catalyst substrate.

The above substrate paste was placed between the two catalyst substratesand passed through a pair of pressure rollers whereby a plate-shapedcatalyst of 1.2 mm in thickness consisting of titanium oxide, molybdateoxide, and vanadium oxide was obtained. It was air-dried for 12 hours inthe atmosphere and calcined at 500° C. for two hours.

It was subjected to water-repellent treatment by spraying it withfluorine resin aerosol (manufactured by 3M) and drying it at 120° C. forone hour. The amount of this fluorine resin coated, as determined fromthe increase of the weight, was about 0.5% of the weight of thecatalyst.

EXAMPLE 2

2.5 kg of ammonium molybdate ((NH₄)₆ Mo₇ O₂₄ /4H₂ O)), 2.33 kg ofammonium methavanadate, 3.0 kg of oxalic acid and water were added to 20kg of titanium oxide powder, and these were kneaded in a kneader toprepare a paste which was then granulated into a tube of 3 mm indiameter. It was then dried in a fluidized-bed dryer, calcined at 550°C. for two hours, and then ground in a hammer mill whereby catalystpowder containing 50% or more of 1 μm or less particles was obtained.

A suspension of a fluorine resin in water (trade name: Polyflon TFEdispersion, produced by Daikin Industries, Ltd.) in an amount of 1%relative to the catalyst was added to 10 kg of catalyst powder andkneaded to give a paste with a water content of 60%.

The resulting paste was coated by a brush on a plate-shaped catalystprepared in the same manner as in Example 1 and dried to be subjected towater-repellent treatment. The thickness of its water-repellent coatingwas 0.05 mm.

EXAMPLE 3

Water was added to a fluorine resin suspension (trade name: Polyflon TFEdispersion, produced by Daikin Industries, Ltd.) and kneaded to give aslurry with a water content of 80%.

The plate-shaped catalyst prepared in the same manner as in Example 1was immersed in the resulting slurry, then deprived of the fluidpresent, and dried whereby a water-repellent coating layer was formed.The thickness of the water-repellent coating layer was about 0.05 mm.

EXAMPLE 4

The plate-shaped catalyst was subjected to water-repellent treatment inthe same manner as in Example 3 except that an acrylic resin emulsion(aqueous acrylic paint, produced by Kampe K. K.) was used in place ofthe fluorine resin suspension.

The thickness of the water-repellent coating layer thus obtained wasabout 0.05 mm.

EXAMPLE 5

A catalyst was obtained in the same manner as in Example 1 except thatthe calcined plate-shaped catalyst while moistened with water wassprayed with the fluorine resin aerosol.

EXAMPLE 6

Water and a fluorine resin suspension (trade name: Polyflon TFEdispersion, produced by Daikin Industries, Ltd.) in an amount of 1%relative to the catalyst were added to 10 kg of fine powder of siliconoxide (trade name: Mycon F, produced by Tomita Seiyaku K. K.), and thesewere kneaded to give a paste with a water content of 60%. It was coatedon a plate-shaped catalyst prepared in the same manner as in Example 2and dried to be subjected to water-repellent treatment.

Comparative Example 1

A plate-shaped catalyst was obtained in the same manner as in Example 1except that it was not coated with the water-repellent resin slurry.

Comparative Example 2

In Example 1, water was added to fine powder of silicon oxide (tradename: Mycon F, produced by Tomita Seiyaku K. K.) and kneaded to preparea slurry with a water content of 60% which was then coated on thesurface of a plate-shaped catalyst substrate prepared in the same manneras in Example 2 to give a catalyst.

Test Example 1

The following accelerated-deterioration test, in which each of theplate-shaped catalysts obtained in Examples 1 to 6 and ComparativeExamples 1 to 2 was placed for a predetermined period in an alkali saltunder conditions causing the salt to deliquesce, was performed toexamine the change of the performance of the catalyst with the amount ofthe alkali adhering to the catalyst.

The plate-shaped catalyst was embedded in a heavy oil burning ashcontaining 20% sodium sulfate and maintained in saturated steam at 60°C. for 6 hours, assuming the time of stating the boiler. To remove theash adhered to the surface of the catalyst and the salt deliquescedafter the test, the catalyst was dried at 120° C., and thereafter adenitration test was conducted under the conditions in Table 1 todetermine the degree of denitration. Further, each catalyst was groundand analyzed for fluorescent X-ray to examine the content of sodiumpenetrated into the catalyst. The results are collectively shown inTable 2.

                  TABLE 1                                                         ______________________________________                                        NO:                    2000 ppm                                               CO.sub.2 :             6                                                      O.sub.2 :              10%                                                    H.sub.2 O:             6%                                                     N.sub.2 :              Balance                                                Gas amount:            3 L/min                                                Area rate:             17 m/h                                                 Reaction temperature:  200° C.                                         ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                           Degree of                                                                     Denitration                                                        Initial Degree                                                                           after                                                              of Denitration                                                                           Deterioration                                                                           Na Content                                               (%)        Test (%)  (wt %)                                           ______________________________________                                        Ex. 1     83.5         77.2      0.1                                          2         85.5         78.3      0.2                                          3         83.2         78.3      0.1                                          4         82.5         77.0      0.1                                          5         83.6         76.8      0.2                                          6         81.5         75.9      0.1                                          Com. Ex. 1                                                                              87.0         55.7      5.8                                          2         83.0         54.2      4.7                                          ______________________________________                                    

As is evident from the results in Table 2, the initial activity of thecatalysts of the present invention (Examples 1 to 6) was slightlyreduced by coating the water-repellent coating layer, but the activityreduction due to the accelerated-deterioration test was low, indicatingtheir superiority in alkali resistance.

Further, the activity of the catalysts in Comparative Examples 1 to 2after the test was greatly reduced, and a large amount of sodium wasdetected in the catalysts. To the contrary, the catalysts of theinvention did not changed the color of the surface even after the testand sodium was hardly detected in the catalysts. It can be seen that inthe catalysts of the invention, the water-repellent coating layer formedon the surface thereof repels sodium sulfate deliquesced, thuspreventing it from entering the catalysts thereby preventing thedeterioration of the catalysts.

EXAMPLES 7 AND 8

2.5 kg of ammonium molybdate ((NH₄)₆ /Mo₇₀₂₄ /4H₂ O)), 2.33 kg ofammonium methavanadate, 3.0 kg of oxalic acid, and 4.8 kg of inorganicfibers (trade name: Kaowool) and water were added to 20 kg of titaniumoxide powder and kneaded in a kneader to prepare a substrate paste witha water content of 33%.

On the other hand, a network material having 1400 twisted filaments of Eglass fibers with a fiber diameter of 9 μm plain-woven at a roughness of10 fibers/inch was impregnated with a slurry consisting of 40% oftitania, 20% of silica sol and 1% of polyvinyl alcohol, and dried at150° C. to make it rigid to give a catalyst substrate.

The above substrate paste was placed between the two catalyst substratesand passed through a pair of pressure rollers whereby a plate-shapedcatalyst of 1.2 mm in thickness consisting of titanium oxide, molybdateoxide, and vanadium oxide was obtained. It was air-dried for 12 hours inthe atmosphere and calcined at 500° C. for two hours to give a moldedbody of the denitration catalyst.

Separately, a fluorine resin dispersion (trade name: Polyflon TFEdispersion, produced by Daikin Industries, Ltd.) was mixed with water toprepare a fluid containing 20% of fluorine resin, and the resultingfluid was coated by a brush on the surface of the above molded body ofthe denitration catalyst and dried at 120° C. for one hour whereby aprimary coating layer was formed. Thereafter, the fluorine resindispersion and water were mixed to prepare a fluid containing 60% offluorine resin (Example 7) and a fluid containing 50% of fluorine resin(Example 8) respectively, and these fluids were coated respectively by abrush on the surface of the denitration catalyst having the primarycoating formed thereon, and then dried at 120° C. for one hour andcalcined at 250° C. for one hour. The total amounts of the fluorineresin coated, as determined from the increase of the weight, were 150g/m² and 140 g/m² respectively.

EXAMPLE 9

Water-repellent treatment was conducted in the same manner as in Example7 except that after the primary coating layer was formed, it was driedat 120° C. for one hour and then calcined at 250° C. for one hour. Theamount of this fluorine resin coated was 150 g/m².

EXAMPLE 10

The molded body of the denitration catalyst obtained in Example 7 wasimmersed in a fluid containing 20% of fluorine resin prepared by mixinga fluorine resin dispersion (trade name: Polyflon TFE dispersion,produced by Daikin Industries, Ltd.) with water, then deprived of thefluid present, and dried whereby a water-repellent primary coating layerwas formed. Thereafter, the catalyst having the primary coating layerformed thereon was immersed in a fluid containing 50% of fluorine resinprepared by mixing the fluorine resin dispersion with water, thendeprived of the fluid present, and calcined at 250° C. for one hour togive a catalyst. The amount of the fluorine resin coated in this casewas 160 g/m².

Comparative Example 3

A fluid containing 30% of fluorine resin prepared by mixing a fluorineresin suspension (trade name: Polyflon TFE dispersion, produced byDaikin Industries, Ltd.) with water was coated by a brush on the surfaceof a molded body of a denitration catalyst prepared in the same manneras in Example 7, then dried and calcined at 250° C. for one hour to givea catalyst. The amount of the fluorine resin coated in this case was 70g/m². This comparative example corresponds to FIG. 4.

Comparative Example 4

A fluorine resin dispersion (Polyflon TFE dispersion, produced by DaikinIndustries, Ltd.) stock solution (containing 60% fluorine resin) wascoated by a brush on the surface of a molded body of a denitrationcatalyst prepared in the same manner as in Example 7, then dried andcalcined at 250° C. for 1 hour to give a catalyst. The amount of thefluorine resin coated in this case was 150 g/m². This comparativeexample corresponds to FIG. 5.

Comparative Example 5

A fluid containing 30% fluorine resin prepared by mixing a fluorineresin suspension (trade name: Polyflon TFE dispersion, produced byDaikin Industries, Ltd.) with water was coated by a brush on the surfaceof a molded body of a denitration catalyst prepared in the same manneras in Example 1 and dried whereby a water-repellent primary coating wasformed. Thereafter, the same fluid containing 30% of fluorine resin wascoated by a brush on the surface of the catalyst having a primarycoating formed thereon, then dried and calcined at 250° C. for 1 hour togive a catalyst. The amount of the fluorine resin coated in this casewas 150 g/m². This comparative example corresponds to FIG. 6.

Comparative Example 6

A plate-shaped catalyst was obtained without coating the water-repellentresin dispersion in Example 7.

Test Example 2

An accelerated-deterioration test, in which each of the catalystsobtained in Examples 7 to 10 and Comparative Examples 3 to 6 was placedfor a predetermined period in an alkali salt under conditions causingthe salt to deliquesce, was performed to examine the change of theperformance of the catalyst. That is, the catalyst was embedded in heavyoil burning ash containing 20% of sodium sulfate and maintained insaturated steam at 60° C. for 6 hours, assuming the time of starting theboiler.

To remove the ash adhered to the surface of the catalyst and the saltdeliquesced after the test, the catalyst was dried at 120° C. to removethe precipitates, and the degrees of denitration were determined underthe conditions in Table 1 and the results are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                Initial   Degree of                                   First/second            Degree    Denitration                                 Concentrations Coated   of        After                                       in Dispersion  Amount   Denitration                                                                             Deterioration                               (%)            (g/m2)   (%)       Test (%)                                    ______________________________________                                        Ex. 7   20/60      150      82.0    81.2                                      8       20/50      140      81.5    80.3                                      9       20/60      150      81.2    81.3                                      10      20/50      160      82.5    80.9                                      Com. Ex. 3                                                                            30          70      85.0    70.7                                      4       60         150      78.0    77.2                                      5       30/30      150      82.2    72.5                                      6       --         --       87.0    65.7                                      ______________________________________                                    

From Table 3, the denitration catalysts of the present invention(Examples 7 to 10) show a less decrease in the degrees of denitration ofthe accelerated-deterioration test, indicating their superiority todurability in use for a long period of time.

To the contrary, in Comparative Example 3 where the thin fluid wascoated once, the degree of denitration before theaccelerated-deterioration test was high but was greatly reduced afterthe accelerated-deterioration test. Further, in Comparative Example 4where the thick fluid was coated once, the degree of denitration wasless changed before and after the accelerated-deterioration test, butwas lower than in Comparative Example 6 where the water-repellenttreatment was not conducted. Further, even in Comparative Example 5where the thin fluid was coated twice, the degree of denitration wasgreatly reduced after the accelerated-deterioration test. In ComparativeExample 6, the water-repellent treatment was not conducted, so thedegree of denitration was high before the accelerated-deteriorationtest, but was considerably reduced after the accelerated-deteriorationtest.

Industrial Applicability

The denitration catalyst of the present invention, even when used in thetreatment of an exhaust gas containing ash containing highlydeliquescent salts, enables the water-repellent porous coating layerformed on the surface to prevent the salts having deliquesced fromentering the catalyst, thus preventing the deterioration of the catalystand enables a high catalytic activity to be maintained for a long periodof time.

In particular, the denitration catalyst obtained by coating the surfaceof the denitration catalyst with a low-concentration dispersion, dryingthe coating, further coating a high-concentration dispersion, and dryingthe coating bears on the surface thereof a uniform, dense and thinwater-repellent porous coating layer, thus enabling significantimprovement of the durability of the denitration catalyst.

Effective denitration of nitrogen oxides (NOx) in an exhaust gascontaining ash with deliquescent salts, such as in an oil-fired boilerexhaust gas, an exhaust gas from a boiler using wood chips and peat asfuels and a furnace exhaust gas can thereby be made feasible, so theindustrial and social worth of the present invention is significantlyhigh.

What is claimed is:
 1. A denitration catalyst for use in the reduction,with ammonia, of nitrogen oxides contained in an exhaust gas containingdeliquescent salts as dust, which bears on the surface of thedenitration catalyst a porous coating layer of a mixture of awater-repellent organic resin with inorganic oxide particles, or aporous coating layer of a mixture of a water-repellent organic resinwith catalyst component particles.
 2. A denitration catalyst accordingto claim 1, wherein the water-repellent organic resin is a resinselected from the group consisting of flourine resin, polyamide resin,acrylic resin and silicon resin.
 3. A denitration catalyst according toclaim 1, wherein the catalyst component comprises titanium oxide, eithermolybdenum oxide or tungsten oxide, and vanadium oxide.
 4. A denitrationcatalyst according to claim 3, wherein the denitration catalyst is aplate-shaped molded body comprising a composition of titanium oxides,either molybdenum oxide or tungsten oxide, and vanadium oxide, filledwithin network inorganic fiber substrates.
 5. A denitration catalystaccording to claim 1, wherein the denitration catalyst is a catalystmolded body containing, titanium oxide, either molybdenum oxide ortungsten oxide, and vanadium oxide.
 6. A denitration catalyst accordingto claim 2, wherein the catalyst component comprises titanium oxide,either molybdenum oxide or tungsten oxide, and vanadium oxide.
 7. Aprocess for preparing a denitration catalyst, which comprises permittingan aqueous slurry or emulsion of at least one resin composition selectedfrom the group consisting of a mixture of the water-repellent organicresin and inorganic oxide particles, and a mixture of thewater-repellent organic resin and catalyst component particles to becoated on, or to adhere by a spray to the surface of a denitrationcatalyst and then evaporating its water to form a porous coating layer.8. A process for preparing a denitration catalyst according to claim 7,wherein the denitration catalyst is a catalyst molded body comprising acomposition of titanium oxide, either molybdenum oxide or tungstenoxide, and vanadium oxide, and the catalyst molded body is previouslymoistened to form the porous coating layer in a wet state.
 9. A processfor preparing a denitration catalyst, which comprises coating an aqueousdispersion containing a water-repellent organic resin at a lowconcentration onto the surface of a denitration catalyst for use in theammonia reduction of nitrogen oxides contained in an exhaust gascontaining highly deliquescent salts as dust, drying the coating,further coating the dried coating with an aqueous dispersion containinga water-repellent organic resin at a higher concentration than the aboveconcentration, and then drying the coating to form a porous coatinglayer of the water-repellent organic resin.
 10. A process for preparinga denitration catalyst according to claim 9, wherein the concentrationof the water-repellent organic resin in the low-concentration aqueousdispersion ranges from 5 to 30% by weight.
 11. A process for preparing adenitration catalyst according to claim 10, wherein the water-repellentorganic resin is a resin selected from the group consisting of flourineresin, polyamide resin, acrylic resin and silicon resin.
 12. A processfor preparing a denitration catalyst according to claim 10, wherein thedenitration catalyst is a catalyst molded body comprising a compositionof titanium oxide, either molybdenum oxide or tungsten oxide, andvanadium oxide.
 13. A process for preparing a denitration catalystaccording to claim 9, wherein the concentration of the water-repellentorganic resin in the low-concentration aqueous dispersion ranges from 10to 20% by weight.
 14. A process for preparing a denitration catalystaccording to claim 9, wherein the concentration of the water-repellentorganic resin in the high-concentration aqueous dispersion ranges from30 to 60% by weight.
 15. A process for preparing a denitration catalystaccording to claim 9, wherein the water-repellent organic resin is aresin selected from the group consisting of flourine resin, polyamideresin, acrylic resin and silicon resin.
 16. A process for preparing adenitration catalyst according to claim 9, wherein the denitrationcatalyst is a catalyst molded body comprising a composition of titaniumoxide, either molybdenum oxide or tungsten oxide, and vanadium oxide.17. A denitration catalyst according to claim 9, wherein the denitrationcatalyst is a plate-shaped molded body comprising a composition oftitanium oxide, either molybdenum oxide or tungsten oxide, and vanadiumoxide, filled between network inorganic fiber substrates.
 18. Adenitration catalyst prepared by a process according to claim 9.